The present invention relates, in general, to microelectromechanical structures (MEMS), such as torsional cantilevers, to actuators for MEMS devices, and to methods for fabricating such structures. More particularly, the invention relates to high aspect ratio, single crystal silicon MEMS devices, such as a torsional cantilever having a cantilever moment arm mounted on a torsional support beam, to actuators incorporating multiple interleaved comb drive fingers which are energizable to actuate the cantilever, to a novel actuator structure, and to a process for fabricating such structures which is compatible with processes for making conventional silicon integrated circuits.
Since their development in the early 1980s, scanning probe microscopes have become important tools for surface analysis and surface modification. The unique applications of scanning probe microscopes include imaging and manipulating single atoms, measuring forces on the atomic scale, and performing nm-scale lithography. At the center of the family of scanning probe microscopes are the scanning tunnelling microscope (STM) and the atomic force microscope (AFM). These macroscopic scanned probe instruments, for the most part, use large piezoelectric actuators to position a sensing tip or a probe in three dimensions (xyz) and thus to provide relative motion between the tip and a sample surface. However, the size of these microscopic instruments limits their performance, for the mass of the tip-actuator structure produces low resonant frequencies and low scanning rates. More importantly, these large instruments cannot be easily integrated into arrays for high speed scanning and atom manipulation, for information storage, or for high throughput, nm-scale lithography systems.
A cantilever of some sort is often used in macroscopic force microscopy to monitor the variations in forces which represent the interaction between a tip and a sample. In such cases, the cantilever is usually a silicon nitride "V" cantilever which, for example, may be 0.6 to 2 micrometers thick, may be 100 to 200 micrometers long, and which may have a spring constant of between about 0.03 and 3 N/m for contact mode imaging. See, for example, T. R. Albrecht et al., J. Vac. Sci. Technol. A8, 3386 (1990). For non-contact mode imaging, the cantilever may be silicon with a spring constant of about 1 to 100 N/m, as described by Wolter et al., J. Vac. Sci. Technol. B9, 1353 (1991). In order to obtain a high degree of sensitivity, a low spring constant k and a high Q is needed for such cantilevers, and attempts have been made to accomplish this through the use of thin films. However, it is necessary to make the cantilever very thin in order to achieve a low spring constant with a thin film; for example, magnetic resonance force detection has been performed using a cantilever (without an integrated tip) that was only 900 .ANG. thick, and which had a spring constant of 10.sup.-3 N/m. However, the fabrication and use of such thin cantilevers poses many problems, including the problem of tip integration, problems with internal stress, and problems in making electrical connections and in amplifying the resulting signals.
For many years, torsion has been used as a technique for highly sensitive measurements of force interactions; for example, measurement of Coulomb's torsional balance for electrostatic forces and Cavendish's balance for gravitation. Furthermore, torsional resonators have been used as high-Q resonators to study a variety of physical properties, such as dissipation and visoelasticity. Such devices have been widely used because cantilevers, resonators, or balances can be made symmetric with respect to their center of mass, with the result that lateral vibrations of the support do not couple to the torsional mode of the sensor device. In the case where the measuring device includes a spring, lateral modes of the spring can be made much stiffer than the torsional mode without affecting the torsional behavior, thus making torsional measurements desirable.
Torsional cantilevers are known to provide a viable option in scanning force microscopy. However, such cantilevers have, in the past, been assembled by hand, using cleaved pieces of silicon, carbon fibers or tungsten wire, and epoxy. Such devices did not incorporate integrated tips and, although microfabricated cantilevers have been demonstrated, such devices exhibited a lateral stiffness which was as soft as, or softer than that of a V-shaped cantilever. The lack of sufficient lateral stiffness can result in unwanted "stick-slip" behavior as the cantilever is scanned across samples.